Use Of Biodegradable Alkoxylated (Meth)Acrylate Copolymers As Crude Oil Demulsifiers

ABSTRACT

The invention relates to the use of copolymers which can be obtained by the polymerization of monomers (A) and (B), (A) being a monomer of formula (I), wherein A represents a C 2  to C 4  alkylene group, B represents a C 2  to C 4  alkylene group different from A, R represents hydrogen or methyl, m is a number from 1 to 500, and n is a number from 1 to 500, and (B) being an ethylenically unsaturated monomer which contains an aliphatic hydrocarbon group, for demulsifying oil/water emulsions in amounts of 0.0001 to 5% by weight, based on the oil content of the emulsion to be demulsified.

The present invention relates to the use of alkoxylated (meth)acrylate copolymers for breaking water-oil emulsions, especially in crude-oil production.

Crude oil is recovered in the form of an emulsion with water. Before further processing the crude oil, these crude-oil emulsions have to be broken to separate them into the oil portion and the water portion. This is generally done using so-called crude-oil or petroleum emulsion breakers, or else “petroleum breakers” for short. Petroleum breakers are surface-active polymeric compounds capable of effectuating the requisite separation in the emulsion constituents within a short time.

It is mainly alkoxylated alkylphenol-formaldehyde resins, nonionic alkylene oxide block copolymers and also variants crosslinked with bisepoxides that are used as demulsifiers. Overviews are given by “Something Old, Something New: A Discussion about Demulsifiers”, T. G. Balson, pp. 226-238 in Proceedings in the Chemistry in the Oil Industry VIII Symposium, 3.-5. Nov. 2003, Manchester, GB, and also “Crude-Oil Emulsions: A State-Of-The-Art Review”, S. Kokal, pp. 5-13, Society of Petroleum Engineers SPE 77497.

U.S. Pat. No. 4,032,514 discloses the use of alkylphenol-aldehyde resins for breaking petroleum emulsions. These resins are obtainable by condensing a para-alkylphenol with an aldehyde, usually formaldehyde.

Such resins are often used in alkoxylated form, as disclosed in DE-A-24 45 873 for example. For this purpose, the free phenolic OH groups are reacted with an alkylene oxide.

In addition to the free phenolic OH groups, free OH groups of alcohols or NH groups of amines can also be alkoxylated, as disclosed in U.S. Pat. No. 5,401,439 for example.

By way of further petroleum emulsion breakers, U.S. Pat. No. 4,321,146 discloses alkylene oxide block copolymers and U.S. Pat. No. 5,445,765 alkoxylated polyethyleneimines. The disclosed breakers can be used as individual components, in mixtures with other emulsion breakers, or else as crosslinked products.

Alkoxylated dendritic polyesters (dendrimers) are disclosed in DE-A-103 29 723 as petroleum emulsion breakers biodegradable to OECD 306.

DE-A-103 25 198 likewise discloses breakers biodegradable to OECD 306. Alkoxylated, crosslinked polyglycerols are concerned here.

The different properties (e.g., asphaltenes, paraffin and salt contents, chemical composition of the natural emulsifiers) and water fractions of various crude oils make it imperative to further develop the existing petroleum breakers. Particularly a low dosage rate and broad applicability of the petroleum breaker to be used is at the focus of economic and ecological concern as well as the higher effectivity sought. There is further an increasing need for emulsion breakers with good biodegradability and low bioaccumulation as replacements for the now controversial alkylphenol-based products.

It is an object of the present invention to develop novel petroleum breakers which are equivalent or superior to the existing alkylphenol-based petroleum breakers in performance, can be used in lower doses and have better ecological degradability.

Surprisingly, alkoxylated (meth)acrylate copolymers are found to give excellent performance as petroleum breakers at very low dose. They also exhibit distinctly better biodegradabilities (to OECD 306) compared with conventional commercial emulsion breakers.

The invention accordingly provides for the use of copolymers obtainable by polymerization of monomers (A) and (B), wherein

(A) is a monomer of formula (I)

where A is a C₂ to C₄ alkylene group, B is a C₂ to C₄ alkylene group other than A, R is hydrogen or methyl, m is from 1 to 500, n is from 1 to 500, and (B) is an ethylenically unsaturated monomer which contains an aliphatic hydrocarbon group, for breaking oil/water emulsions in amounts of 0.0001% to 5% by weight based on the oil content of the emulsion to be broken.

Copolymers used in a preferred embodiment are obtainable by polymerization of monomers (A), (B) and (C) wherein (C) is an ethylenically unsaturated monomer which contains an aromatic group.

The copolymers being obtainable by the polymerization of monomers (A), (B) and optionally (C) is to be understood as meaning that the copolymers contain structural units derived from the monomers (A), (B) and optionally (C) when these are subjected to free-radical polymerization.

The copolymer according to the invention generally possesses customary terminal groups formed by the initiation of the free-radical polymerization or by chain transfer reactions or by chain termination reactions, for example a proton, a group from a free-radical initiator or a sulfur-containing, for example, group from a chain transfer reagent.

The molar fraction of the monomers in a preferred embodiment is from 0.1% to 99.9% for monomer (A) and from 0.1% to 99.9% for monomer (B), more particularly from 1% to 99.5% for monomer (A) and from 0.5% to 99% for monomer (B), and specifically from 10% to 90% for monomer (A) and from 10% to 90% for monomer (B).

The molar fraction of the monomers in a further preferred embodiment is from 1% to 80% for monomer (A), from 0.1% to 80% for monomer (B) and from 0.1% to 80% for monomer (C), and more particularly from 10% to 70% for monomer (A), from 10% to 60% for monomer (B) and from 1% to 60% for monomer (C).

In a further preferred embodiment, the monomers (A) and (B) add up to 100 mol %.

In a further preferred embodiment, the monomers (A), (B) and (C) add up to 100 mol %.

The alkylene oxide units (A-O)_(m) and (B-O)_(n) can form either a random arrangement or, as in the case of a preferred embodiment, a blockwise arrangement.

In a preferred embodiment, (A-O)_(m) represents a block of propylene oxide units and (B-O)_(n) represents a block of ethylene oxide units, or (A-O)_(m) represents a block of ethylene oxide units and (B-O)_(n) represents a block of propylene oxide units, wherein the molar fraction of ethylene oxide units is preferably from 50% to 98%, more particularly from 55% to 95% and more preferably from 60% to 93%, based on the sum total (100%) of the ethylene oxide and propylene oxide units.

m is preferably from 2 to 50. n is preferably from 2 to 50.

The number of alkylene oxide units (n+m) is preferably from 2 to 500, more particularly from 4 to 100 and more preferably from 5 to 80.

Preferred monomers (B) conform to formula (II)

where R¹ is hydrogen or methyl, Y is a linear, branched or cyclic aliphatic hydrocarbon radical of 1 to 30 carbon atoms, preferably 4 to 28 carbon atoms and particularly 6 to 24 carbon atoms, which may be saturated or unsaturated, and which may contain heteroatoms selected from O, N and S, W_(a) is oxygen or the group —NH—.

The monomers (B) preferably include esters (Wa=oxygen) and amides (W_(a)=—NH—) of acrylic acid and of methacrylic acid in each of which Y represents the following groups: methyl, ethyl, propyl, isopropyl, n-butyl, isobutyl, t-butyl, pentyl, hexyl, 2-ethylhexyl, 3,3-dimethylbutyl, heptyl, octyl, isooctyl, nonyl, lauryl, cetyl, stearyl-behenyl, cyclohexyl, trimethylcyclohexyl, t-butylcyclohexyl, bornyl, isobornyl, adamantyl, (2,2-dimethyl-1-methyl)propyl, cyclopentyl, 4-ethyl-cyclohexyl, 2-ethoxyethyl, tetrahydrofurfuryl and tetrahydropyranyl and also the alkyl chain cuts C_(16/18) or C_(20/24).

Preferred monomers (B) are the following alkyl esters and/or alkylamides of acrylic acid and methacrylic acid: methyl, ethyl, propyl, butyl, isobutyl, 2-ethoxyethyl, myristyl, octadecyl, and more preferably 2-ethylhexyl, lauryl and stearyl.

Preferred monomers (C) conform to formulae (IIIa) or (IIIb):

where X_(a) is an aromatic or araliphatic radical of 3 to 30 carbon atoms, which may optionally contain heteroatoms selected from O, N and S, Z_(a) is H or C₁ to C₄ alkyl, Z_(b) is H or C₁ to C₄ alkyl, and Z_(c) is H or C₁ to C₄ alkyl;

where R² is hydrogen or methyl, X_(b) is an aromatic or araliphatic radical of 3 to 30 carbon atoms, which may optionally contain heteroatoms selected from O, N and S, W_(b) is oxygen or the group —NH—.

The monomers (C) preferably include esters (W_(a)=oxygen) and amides (W_(a)=—NH—) of acrylic acid and methacrylic acid, in each of which X_(a) or X_(b) represents phenyl, benzyl, tolyl, 2-phenoxyethyl or phenethyl groups.

Preferably, X_(a) or X_(b) represent aromatic or araliphatic radicals of 6 to 24 carbon atoms.

Further suitable monomers (C) are vinylaromatic monomers such as styrene and its derivatives such as for example vinyltoluene and alpha-methylstyrene. The aromatic unit may also comprise heteroaromatics, for example 1-vinylimidazole.

Particularly preferred monomers (C) are styrene, 1-vinylimidazole, benzyl (meth)acrylate, 2-phenoxyethyl (meth)acrylate and phenethyl (meth)acrylate.

The copolymers according to the invention have a molecular weight of preferably 10³ g/mol to 10⁹ g/mol, more preferably of 10³ to 10⁷ g/mol and more particularly 3·10³ to 10⁵ g/mol.

The copolymers according to the invention are obtainable via free-radical polymerization. The polymerization reaction can be carried out continuously, batchwise or semi-continuously.

The polymerization reaction is preferably carried out as a precipitation polymerization, emulsion polymerization, solution polymerization, bulk polymerization or gel polymerization. Solution polymerization is particularly advantageous for the performance profile of the copolymers according to the invention.

Useful solvents for the polymerization reaction include all organic or inorganic solvents that behave very substantially inertly with respect to free-radical polymerization reactions, examples being ethyl acetate, n-butyl acetate or 1-methoxy-2-propyl acetate, and also alcohols such as, for example ethanol, isopropanol, n-butanol, 2-ethylhexanol or 1-methoxy-2-propanol, and also diols such as ethylene glycol and propylene glycol. It is also possible to use ketones such as acetone, butanone, pentanone, hexanone and methyl ethyl ketone, alkyl esters of acetic, propionic and butyric acids such as, for example, ethyl acetate, butyl acetate and amyl acetate, ethers such as tetrahydrofuran, diethyl ether and ethylene glycol monoalkyl ether, ethylene glycol dialkyl ether, polyethylene glycol monoalkyl ether, polyethylene glycol dialkyl ether. It is similarly possible to use aromatic solvents such as, for example, toluene, xylene or higher-boiling alkylbenzenes. It is likewise conceivable to use solvent mixtures, in which case the choice of solvent or solvents depends on the planned use of the copolymer according to the invention. Preference is given to using water; lower alcohols; preferably methanol, ethanol, propanols, isobutanol, sec-butanol, t-butanol, 2-ethylhexanol, butylglycol and butyldiglycol, more preferably isopropanol, t-butanol, 2-ethylhexanol, butylglycol and butyldiglycol; hydrocarbons of 5 to 30 carbon atoms and mixtures and emulsions thereof.

The polymerization reaction is preferably carried out in the temperature range between 0 and 180° C. and more preferably between 10 and 100° C., not only at atmospheric pressure but also under elevated or reduced pressure. Optionally, the polymerization can also be carried out under a protective gas atmosphere, preferably under nitrogen.

The polymerization can be initiated using high-energy electromagnetic rays, mechanical energy or the customary, chemical polymerization initiators such as organic peroxides, e.g., benzoyl peroxide, tert-butyl hydroperoxide, methyl ethyl ketone peroxide, cumoyl peroxide, dilauroyl peroxide (DLP) or azo initiators, for example azobisisobutyronitrile (AIBN), azobisamidopropyl hydrochloride (ABAH) and 2,2′-azobis(2-methylbutyronitrile) (AMBN). It is likewise possible to use inorganic peroxy compounds, for example (NH₄)₂S₂O₈, K₂S₂O₈ or H₂O₂, optionally combined with reducing agents (e.g., sodium hydrogensulfite, ascorbic acid, iron(II) sulfate) or redox systems which contain an aliphatic or aromatic sulfonic acid (e.g., benzenesulfonic acid, toluenesulfonic acid) as reducing component.

The usual compounds are used as molecular weight regulators. Suitable known regulators include for example alcohols, such as methanol, ethanol, propanol, isopropanol, n-butanol, sec-butanol and amyl alcohols, aldehydes, ketones, alkylthiols, for example dodecylthiol and tert-dodecylthiol, thioglycolic acid, isooctyl thioglycolate and some halogen compounds, for example carbon tetrachloride, chloroform and methylene chloride.

A preferred aspect of the present invention is the use of the alkoxylated (meth)acrylate copolymers as breakers for oil/water emulsions in petroleum recovery.

An important criterion for a petroleum breaker is its water number, which describes the partitioning of the breaker into an aqueous and an organic phase, and thereby is a direct measure of the HLB value of the petroleum breaker. Water number can be used to classify petroleum breakers as more lipophilic or as more hydrophilic. The higher the water number, the greater the hydrophilicity of the compound in question. Water number is determined by dissolving one gram of the petroleum breaker in 30 mL of 97:3 (v/v) dioxane/toluene. This solution is admixed with water until persistent cloudiness occurs. Water number is the amount of water added in mL. Water numbers from 22.0 mL are an upper limit for this method of determination.

The water number of the petroleum breaker has significant influence on its properties. The greater the water number, the earlier the breaking of the petroleum emulsion occurs in most cases, although the breaking tends to be incomplete, i.e., oil remains in the water separated off. The smaller the water number, the slower the petroleum emulsion breaks in most cases, but the breaking tends to be complete, i.e., only very little oil remains in the water separated off. Owing to the various characteristics of crude-oil emulsions, it is important to have a wide range of petroleum breakers in terms of water number. Water number is preferably at least 5. Water number can itself be influenced through the amount of EO (water number increases with increasing EO content), PO (water number decreases with increasing PO content), the size of hydrocarbon radicals (water number decreases with increasing number of carbon atoms) and the molecular weight of the polymer (water number decreases with'increasing molecular weight).

When used as petroleum breakers, the alkoxylated (meth)acrylate copolymers are added to the water-in-oil emulsions, preferably in solution. Alcoholic solvents are preferred for the alkoxylated (meth)acrylate copolymers. The alkoxylated (meth)acrylate copolymers are used in amounts of 0.0001% to 5%, preferably 0.0005% to 2%, especially 0.0008% to 1% and specifically 0.001% to 0.1% by weight, based on the oil content of the emulsion to be broken.

EXAMPLES General Polymerization Prescription 1 (Copolymer of Monomers A and B)

A flask equipped with stirrer, reflux condenser, internal thermometer and nitrogen inlet was initially charged with the monomer A, monomer B and optionally the molecular weight regulator in solvent under an incoming flow of nitrogen. Then, under agitation, the temperature was raised to 80° C. and a solution of the initiator was metered during one hour. This was followed by a further 5 hours of stirring at that temperature. The molar mass of the copolymer was analyzed via GPC (reference: polyethylene glycol).

General Polymerization Prescription 2 (Copolymer of Monomers A, B and C)

A flask equipped with stirrer, reflux condenser, internal thermometer and nitrogen inlet was initially charged with the monomer A, monomer B, monomer C and optionally the molecular weight regulator in solvent under an incoming flow of nitrogen. Then, under agitation, the temperature was raised to 80° C. and a solution of the initiator was metered during one hour. This was followed by a further 5 hours of stirring at that temperature and then the solvent was removed under reduced pressure. The molar mass of the copolymer was analyzed via GPC (reference: polyethylene glycol).

The tables which follow contain synthesis examples in which the polymer was prepared according to synthesis prescription 1 or 2.

Equivalents of monomer A PO EO Equivalents of Equivalents of Regulator Water Example (mean MW) number number monomer B monomer C yes(+)/no(−) number Mw 1 1.5 (350)   2 3 1 (LMA) 1 (BMA) + 8.6 5800 2 1 (350) 2 3 1 (LMA) — − 8.4 10600 3 1 (350) 2 3 1 (LMA) — + 8.4 6100 4 2 (350) 2 3 1 (EHMA) 1 (styrene) + 8.8 6200 5 2 (350) 2 3 1 (SA) — − 7.9 9900 6 2 (350) 2 3 1 (SA) 1 (styrene) − 8.8 9300 7 1 (500) 2 7 1 (LMA) — + 11.8 6100 8 2 (500) 2 7 1 (LMA) 1 (styrene) + 12.2 6200 9 1 (500) 2 7 1 (EHMA) — + 11.6 6800 10 1 (600) 2 9 1 (LMA) — + 14.4 6500 11 1 (600) 2 9 1 (EHMA) — + 14.6 6600 12 1 (750) 2 13 1 (SA) — − 20.0 12000 13 1 (750) 2 13 1 (SA) 1 (styrene) − >22.0 14000 14 1 (750) 2 13 2 (LMA) 2 (styrene) − 16.8 7900 15 1 (750) 2 13 1 (MMA) — − >22.0 8200 16 1 (750) 2 13 2 (MMA) 2 (styrene) − 18.7 8500 17 1 (750) 2 13 1 (CMA) 1 (BMA) + 17.9 6600 18 1 (750) 2 13 1 (CMA) 1 (styrene) + 18.2 6900 19 1 (850) 2 15 1 (EHMA) — + >22 6900 20 1 (850) 2 15 2 (LMA) 2 (styrene) − 18.1 11400 21 1 (750) 5 7 1 (LMA) 1 (BMA) + 17.1 6800 22 1 (750) 5 7 2 (LMA) 2 (styrene) − 14.4 9400 23 1 (750) 5 7 1 (EHMA) — + 18.2 5900 24 2 (750) 5 7 1 (EHMA) 2 (styrene) + 18.4 6200 25 2 (750) 5 7 1 (CMA) — − 19.2 9800 26 1 (750) 5 7 1 (SA) 1 (styrene) − 18.3 11000 27  1 (1000) 5 13 1 (LMA) — + >22.0 7200 28  1 (1000) 5 13 2 (EHMA) 2 (styrene) + 19.7 7400 29  1 (1000) 10 6 1 (MMA) — + 16.4 8000 30  1 (1000) 10 6 1 (MMA) 1 (styrene) − 15.9 15000 31  1 (2000) 10 29 2 (SA) 2 (BMA) + >22.0 8200 32  1 (2000) 10 29 2 (SA) 2 (styrene) + >22.0 8400 MW = average molecular weight PO = propylene oxide EO = ethylene oxide Mw = weight average of polymer LMA = lauryl methacrylate EHMA = 2-ethylhexyl methacrylate SA = stearyl acrylate MMA = methyl methacrylate CMA = cyclohexyl methacrylate BMA = benzyl methacrylate Regulator = dodecanethiol

Determination of Breaking Efficacy of Petroleum Emulsion Breakers

Emulsion breaker efficacy was determined by determining water separation from a crude-oil emulsion per unit time and also the dehydration of the oil. To this end, breaker glasses (conically tapered, graduated glass bottles closeable with a screw top lid) were each filled with 100 ml of the crude-oil emulsion, a defined amount of the emulsion breaker was in each case added with a micropipette just below the surface of the oil emulsion, and the breaker was mixed into the emulsion by intensive shaking. Thereafter, the breaker glasses were placed in a temperature control bath (50° C.) and water separation was tracked.

On completion of emulsion breaking, samples of the oil was taken from the top part of the breaker glass (top oil) and the water content thereof determined to Karl Fischer. In this way, the novel breakers were assessed in terms of water separation and also oil dehydration.

Breaking Effect of Breakers Described

Origin of crude-oil emulsion: Hebertshausen, Germany Water content of emulsion: 48% Demulsifying temperature: 50° C.

The efficacy of the alkoxylated (meth)acrylate copolymers as emulsion breakers compared with Dissolvan® 3567-1c (an alkoxylated alkylphenol resin) at a dose rate of 175 ppm is shown in the following table:

Water Co- Water separation in ml in top polymer per time in min oil Example as per 5 10 20 30 60 90 120 [%] 33 product 1 15 28 34 34 36 36 38 1.0 34 product 2 0 0 4 8 32 34 37 1.2 35 product 7 0 0 5 15 34 42 45 0.5 36 product 10 0 0 0 0 4 11 26 0.3 37 product 14 0 4 26 34 44 44 45 0.9 38 product 19 0 0 0 0.5 4 22 32 1.5 39 Dissolvan 0 0 0.5 0.5 2 10 12 1.5 (comp.) 3567-1c

The biodegradability of the alkoxylated (meth)acrylate copolymers (closed bottle test to OECD 306) compared with standard products is reported in the following table:

Biodegradability in % after Example Copolymer as per 14 days 28 days 40 product 1 17 18 41 product 2 21 24 42 product 7 17 30 43 product 10 20 31 44 product 19 13 30 45 (comp.) Dissolvan 3567-1c (comparator) 6 6 46 (comp.) Reference (sodium benzoate) 74 84 

1. A process for breaking an oil/water emulsion, comprising the step of adding at least one copolymer obtainable by polymerization of monomers (A) and (B), wherein (A) is a monomer of formula (I)

wherein A is a C₂ to C₄ alkylene group, B is a C₂ to C₄ alkylene group other than A, R is hydrogen or methyl, m is from 1 to 500, n is from 1 to 500, and (B) is an ethylenically unsaturated monomer which contains an aliphatic hydrocarbon group, in amounts of 0.0001% to 5% by weight based on the oil content of the emulsion to be broken to the emulsion to be broken.
 2. A process according to claim 1, wherein the at least one copolymer is obtainable by polymerization of monomers (A), (B) and (C) wherein (C) is an ethylenically unsaturated monomer which contains an aromatic group.
 3. A process according to claim 1, wherein the molar fraction of monomer (A) is in the range from 0.1% to 99.9% and of monomer (B) is in the range from 0.1% to 99.9% in the copolymer.
 4. A process according to claim 2, wherein the molar fraction of monomer (A) is from 1% to 80%, of monomer (B) is from 0.1% to 80% and of monomer (C) is from 0.1% to 80% in the copolymer.
 5. A process according to claim 2, wherein the molar fraction of monomer (A) is from 10% to 70%, of monomer (B) is from 10% to 60% and of monomer (C) is from 1% to 60% in the copolymer.
 6. A process according to claim 1, wherein the alkylene oxide units (A-O)_(m) and (B-O)_(n) are arranged blockwise.
 7. A process according to claim 1, wherein (A-O)_(m) represents a block of propylene oxide units and (B-O)_(n) represents a block of ethylene oxide units, or (A-O)_(m) represents a block of ethylene oxide units and (B-O)_(n) represents a block of propylene oxide units, and wherein the molar fraction of ethylene oxide units, based on the sum total of ethylene oxide and propylene oxide units, is from 50% to 98%.
 8. A process according to claim 7, wherein the molar fraction of ethylene oxide units in the copolymer is from 55% to 95%, based on the sum total of ethylene oxide and propylene oxide units.
 9. A process according to claim 1, wherein the monomer (B) is a compound of formula (II):

wherein R¹ is hydrogen or methyl, Y is a linear, branched or cyclic aliphatic hydrocarbon radical of 1 to 30 carbon atoms, preferably 4 to 28 carbon atoms and particularly 6 to 24 carbon atoms, which may be saturated or unsaturated, and which may contain heteroatoms selected from O, N and S, W_(a) is oxygen or the group —NH—.
 10. A process according to claim 2, wherein the monomer (C) is a compound of formulae (IIIa) or (IIIb)

wherein X_(a) is an aromatic or araliphatic radical of 3 to 30 carbon atoms, which may optionally contain heteroatoms selected from O, N and S, Z_(a) is H or C₁ to C₄ alkyl, Z_(b) is H or C₁ to C₄ alkyl, and Z_(c) is H or C₁ to C₄ alkyl;

wherein R² is hydrogen or methyl, X_(b) is an aromatic or araliphatic radical of 3 to 30 carbon atoms, which may optionally contain heteroatoms selected from O, N and S, W_(b) is oxygen or the group —NH—.
 11. A process according to claim 1, wherein said monomer (B) is an alkyl ester or alkylamide of acrylic or methacrylic acid and wherein alkyl has the meaning methyl, ethyl, propyl, butyl, isobutyl, 2-ethylhexyl, 2-ethoxyethyl, myristyl, lauryl and stearyl.
 12. A process according to claim 2, wherein said monomer (C) is styrene, 1-vinylimidazole, benzyl (meth)acrylate, 2-phenoxyethyl (meth)acrylate or phenethyl (meth)acrylate.
 13. A process according to claim 1, wherein m is from 2 to
 50. 14. A process according to claim 1, wherein n is from 2 to
 50. 15. A process according to claim 10, wherein X_(a) or X_(b) represent aromatic or araliphatic radicals of 6 to 24 carbon atoms.
 16. A process according to claim 1, wherein (n+m) is from 5 to
 80. 17. A process according to claim 4, wherein the molar fraction of monomer (A) is from 10% to 70%, of monomer (B) is from 10% to 60% and of monomer (C) is from 1% to 60% in the copolymer. 